What was found
(CAN) CREW REPORTED DEFLECTION OF RUDDER LEFT TO RIGHT, HAPPENED TWICE IN CRUISE. CLIMB-OUT AT 260 NOTICED SHARP YAW MOVEMENT THEN RETURNED TO NORMAL, TOO QUICK TO NOTICE YAW FLUCTUATION. YAW COUPLER REPLACED. IN CRUISE, AT FL 310, YAW OSCILLATION STARTED, YAW DAMPER DISCONNECTED, OSCILLATION STOPPED, YAW DAMPER COUPLER REPLACED, RUDDER POWER CONTROL UNIT REPLACED.